1. Field of the Invention
The present invention relates to a temperature distribution measuring apparatus for obtaining a temperature distribution along an optical fiber by inputting pulsed-light into the optical fiber and measuring backward Raman-scattered light in the optical fiber caused by the pulsed-light and, more particularly, to a temperature distribution measuring apparatus for obtaining improved measurement precision by removing periodic noise caused by clocks of a digital system in the measurement apparatus.
2. Description of the Related Art
In the field of optical application sensing, an Optical Time Domain Reflectometry (OTDR) type temperature distribution measuring apparatus is known. Such a measuring apparatus obtains a temperature distribution along an optical fiber by inputting pulsed-light from one end of the optical fiber, and sampling and measuring backward Raman-scattered light scattered at respective points in the optical fiber.
FIG. 1 is a block diagram showing the overall arrangement of a conventional OTDR temperature distribution measuring apparatus. A light source 10 for emitting pulsed-light is connected to an optical fiber probe 14 for temperature measurement via a directional coupler 12. The probe 14 is a graded-index (GI) type optical fiber. The probe 14 is set in a place where the temperature is to be measured. In FIG. 1, the probe 14 is looped. However, the probe 14 need only be arranged along an object to be measured, and need not always be looped.
Pulsed-light emitted from the light source 10 is input to the probe 14 via the directional coupler 12, is Raman-scattered in the probe 14, and returns as backward scattered light. The backward scattered light is input to an optical filter 16 through the directional coupler 12. The optical filter 16 includes two filters for respectively extracting anti-Stokes' light and Stokes' light from the incident backward Raman-scattered light. The anti-Stokes' light and Stokes' light have different wavelengths, and can be extracted by inserting filters for the corresponding wavelengths.
The anti-Stokes' light and Stokes' light are respectively input to light-receiving elements 18 and 20, and are converted into electrical signals representative of the intensities of the anti-Stokes' light and the Stokes' light. These electrical signals are amplified by amplifiers 22 and 24. The analog electrical signals output from the amplifiers 22 and 24 are sampled by A/D converters 26 and 28 at high speed. Each of the A/D converters 26 and 28 is normally constituted by a plurality of A/D conversion circuits since it must execute high-speed sampling. The high-speed sampling can be realized by sequentially driving the plurality of A/D conversion circuits by shifting their sampling periods by a time obtained by dividing each sampling period by the number of A/D conversion circuits. Digital signals obtained by the A/D converters 26 and 28 are supplied to a signal processor 30, and the signal processor 30 calculates a temperature distribution on the basis of the ratio between the two digital signals. Note that the signal processor 30 supplies a driving signal for controlling the light emission timing to the light source 10.
Of backward Raman-scattered light, the intensity of the anti-Stokes' light changes depending on the temperature, while a temperature-dependent change in intensity of the Stokes' light is not as large. More specifically, theoretically speaking, the temperature is expressed as a function of the ratio of the intensity of the anti-Stokes' light to intensity of the Stokes' light. The time from when pulsed-light is generated until backward scattered light is observed is proportional to the distance between the light source 10 and the scattered position of the pulsed-light if the light velocity in the optical fiber 14 is assumed to be constant. Therefore, when the ratio the intensity of anti-Stokes' light to the intensity of the Stokes' light is plotted along the time axis, the temperature distribution along the probe 14 can be measured.
When temperature measurement is performed based on the above-mentioned method, the obtained signal is very small, and includes many noise components. Thus, by utilizing the fact that the signal includes many random noise components, several ten thousands to several millions of measurements are performed, and the measurement results are averaged by the signal processor 30, thereby removing the effect of random noise components.
However, although this method can remove random noise components having no time correlations, it is very difficult for this method to remove noise components having time correlations, in particular, noise components which are synchronous with pulsed-light such as clocks of digital circuits.
Noise caused by clocks of digital circuits are discussed below. More specifically, the above-mentioned temperature distribution measuring apparatus must execute high-speed signal sampling. For example, in order to perform measurement with a distance resolution of 1 meter, sampling must be performed at 100 MHz. An A/D converter used for such a purpose is called a flash type A/D converter, normally has precision as low as about 8 bits, and has no integral function, as a matter of course. However, the above-mentioned temperature distribution measuring apparatus requires precision as high as 1 bit or less.
Thus, by utilizing the fact that a signal originally includes many random noise components, required resolution is realized by executing several ten thousands to several millions of numerical value averaging processing operations. Such a signal processing method is called a dither method, and can obtain resolution as high as that obtained using a 10- or 12-bit A/D converter even when, e.g., an 8-bit A/D conversion is used.
However, since clock noise of digital circuits is synchronous with sampling, it cannot be removed by the above-mentioned averaging process. The clock noise is mainly generated during the operations of the A/D converters 26 and 28, and is often generated in the light-receiving elements 18 and 20 or the amplifiers 22 and 24 as well. In this case, although clock noise can be slightly decreased by reinforcing a power supply or by grounding, it is difficult to remove all the clock noise.
In the conventional temperature distribution measuring apparatus, anti-Stokes' light and Stokes' light are measured by different electronic circuits. Since synchronous noise patterns caused by clocks vary depending on individual circuits, it is almost impossible, in consideration of characteristic variations of circuit elements, to prepare a plurality of circuits having completely the same synchronous noise patterns.
For this reason, when the ratio of the intensity of Stokes' light to that of anti-Stokes' light is converted into the temperature, periodic noise caused by a difference in synchronous noise patterns in Stokes' light and anti-Stokes' light owing to clocks still remains, and this limits the measurement resolution. The same applies to all periodic noise components owing to factors except for clocks.
Published Unexamined Japanese Patent Application No. 2-145932 describes an apparatus which removes periodic noise. This apparatus obtains a measurement by subtracting a measured value obtained when no pulsed-light is input to an optical fiber from a measured value obtained when pulsed-light is input, thereby detecting periodic noise generated in a measurement system. However, two measurements are required, and the measurement time is doubled. In addition, it is not ensured that the same noise components are generated in the two measurements, and periodic noise cannot be precisely detected when pulsed-light is emitted, since a large current is supplied to a light source, the noise may include switching noise.
As described above, the conventional temperature distribution measuring apparatus has undesirably low detection precision since the obtained signal includes periodic noise components synchronous with measurement, e.g., synchronous with clocks.